1. Field of the Invention
This invention relates to the field of enhancing stereophonic sound reproduction by cross coupling.
2. Description of the Prior Art
The requisite of a stereophonic system is that its sound field must provide the listener with an illusion of spatial dimension giving both a sense of depth and location to the instrument(s) creating it.
In order to create such an illusion, the system must have a minimum of two transducers driven by separate but related signals. When the transducers are loudspeakers, effective operation requires their separation by a distance of 14 at least several feet. This allows the sound from the loudspeakers to be sufficiently spread out so as to be shared by both ears.
This sharing is termed acoustic cross coupling. Not only is it a natural result of the stereo loudspeaker listening experience, it is an essential element.
Unfortunately, acoustic coupling is denied the headphone listener because the sound at each ear is restricted to its own transducer's output. The resultant sound field lacks depth, and instead it is compressed and wedged into the central upper portion of the head.
Conventional stereo system information is composed of two signals, one left and one right channel respectively. Recording these signals using only left and right microphones rarely yields a commercially acceptable product. Further processing is usually required. This consists of adding signal enhancements that simulate the acoustic effects of reverberation and cross coupling.
Reverberation is the term given to the modification of a sound by its own reflections. These come from the surfaces forming the enclosed space in which the sound originates. It differs from an echo by having no discernable time delay between original and reflected sounds. They thus appear to be one.
It is simulated by splitting the signal into two parts. One part passes through a network whose output is delayed in real time. The delayed signal is then frequency contoured and added to the other part of the original signal.
Cross coupling simulation (hereinafter termed cross coupling), is a process previously exclusive to the commercial recording establishment. It is now available to the home listener notably in equipment made by Carver, see, e.g., U.S. Pat. Nos. 4,603,429, 4,309,570 and 4,218,585.
A cross coupled signal is formed by combining a channel's original signal with a fractional part of the signal from its opposite channel. The divisor creating a signal's fractional part is designated as K. This is done simultaneously in both channels.
Here is an example. A two channel stereo system of left signal amplitude L, would be changed to a new signal L-R/K. The term R/K is of course, the fractional part of the system's right signal. The new right signal correspondingly becomes R-L/K.
No presently available system can fully compensate for the effects caused by a lack of acoustic cross coupling. It has been demonstrated however that cross coupled signals are of aid in headphone listening. Most listeners report a marked sense of spatial expansion to the resultant sound field.
There are basically two known ways of doing this.
In the first method, the signals are altered in the amplifier itself prior to going out to the headset. This is so simple and inexpensive to implement that its lack of use defies explanation.
An amplifier is set at unity gain by means of a resistor ra which connects its output to its inverting input. A signal R applied to the inverting input through a series resistor rb, would appear in the output essentially as -R times the quantity ra/ra+rb. (The minus sign results from the use of the inverting input). A signal L applied to the non inverting input would be neither attenuated nor inverted and would therefor be output as L. If both of these signal inputs were concurrently applied, the output would be L-R(ra/ra+rb). By proper design, the quantity (ra/ra+rb) could be used to fractionate R in the same manner as the cross coupling constant K. The amplifier's output could then be the desired cross coupled signal of one channel. Two such circuits would be required for stereo.
A second method of producing cross coupling is to modify the signals at the headset itself. It must first be noted, however, that the patent art does shows some means for achieving acoustic cross coupling in headphones. Most notable is the method proposed in U.S. Pat. No. 4,173,715 to Gosman. He splits each transducer's acoustic output into two parts. One goes directly to the ear at which its transducer resides, and the other portion is fed by a duct to the opposite ear. Through a mechanical design, Gossman allows the ducted sound from one transducer to be mixed with sound from the transducer at that ear. Further, his design avoids the gross bulk that one would perceive as characterizing such a device. Its major disadvantage is its low acoustic cross coupling efficiency. This results in a diminished stereo effect, almost a subliminal quality.
A signal processing method assigned to Koss Corp., and disclosed in U.S. Pat. No. 3,924,072, places resistors in shunt with the audio input as well as resistance in series with the return lead of each transducer. In order to achieve a channel signal to fractional opposite signal ratio of about two to one, the network must dissipate as useless heat, approximately 90 percent of the applied audio power. In a market sustained almost solely by use with portable battery 14 operated amplifiers, such audio power waste translates into rapid battery use, and thereby into a marketing disaster. See also U.S. Pat. No. 3,984,636.
Accordingly, a more cost effective, less power consuming method and apparatus for achieving cross coupling in stereo headphones is needed.